Paraformaldehyde inactivated tumor cells were shown to retain their tumor transplantation antigen (TTA) activity to a degree comparable to that of x-ray inactivated tumor cells. If cells were fixed at lower temperatures below 10 degrees C, the TTA activity was greatly reduced. This process of TTA inactivation at lower temperatures was completely reversible. If cells held at 0 degree C were fixed at 0 degree C, their TTA activity was reduced as described. If, however, the cells were then warmed to above 10 degrees C before fixation, TTA was fully restored. Fixation for longer than one hour or at paraformaldehyde concentrations greater than 1% caused a decrease in immunogenicity. Fixed tumor cells could be stored for periods longer that one month without loss of TTA activity.